Paramagnetic Complexes Research Articles

Proton-detected fast-magic-angle spinning NMR of paramagnetic inorganic solids.

Fast (60 kHz) magic angle spinning solid-state NMR allows very sensitive proton detection in highly paramagnetic organometallic powders. We showcase this technique with the complete assignment of 1H and 13C resonances in a high-spin Fe(ii) polymerisation catalyst with less than 2 mg of sample at natural abundance.

Changes in the Relative Nitric Oxide Content in the Cortex of a Rat Brain in the Acute Ischemia Model

This study uses the electron paramagnetic resonance (EPR) method to study changes in the relative nitric oxygen content in the cortex of a rat brain in conditions of acute ischemia. Laser-induced vascular thrombosis is used as the model of ischemia. Rose bengal is administered into the bloodstream beforehand. In response to laser beaming, it generates reactive oxygen species, which induces protein coagulation, vascular wall damage, and, as a consequence, the formation of thrombus. It is shown histologically that the acute ischemic process ends a day after the laser surgery, and active remission, with the corresponding morphological changes in the area of the lesion, starts within three days. The relative nitric oxide content is determined 1 and 3 days after the brain vascular photothrombosis using the NO–Fe–DETC paramagnetic complex as a spin trap. The nitric oxide content is estimated by the extreme lower field of the EPR spectrum of this complex. It is shown that a significant increase in the relative nitric oxide (NO) content occurs on the third day after laser surgery and coincides with the beginning of remission after the acute ischemia. A possible role of nitric oxide in the remission is discussed in the study.

Bis(p-aminobenzoato)bis(hexadecanoato)dicopper(II): A low-temperature and thermally-stable functional metallomesogen

A novel copper (II) mixed carboxylates namely bis ( p -aminobenzoato) bis (hexadecanoato)dicopper(II), [Cu 2 ( p -H 2 NC 6 H 4 COO) 2 (CH 3 (CH 2 ) 14 COO) 2 ], from one-pot self-assembly of the corresponding ions was designed to function as a low-temperature and thermally-stable metallomesogen applications. The as-prepared copper (II) complex was characterized by means of CHNS analysis, Fourier Transform Infrared (FTIR) spectroscopy, Ultraviolet-Visible (UV-Vis) spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), optical polarized microscopy (OPM), magnetic Gouy balance, and cyclic voltammetry (CV) for the determination of its elemental compositions, chemical boding, optical properties, thermal stability, phase transition, magnetic properties and electrochemical behavior of the complex, respectively. Its melting and decomposition temperatures are 115 o C and 256 o C, respectively, and formed a smectic C mesophase on cooling from the isotropic liquid phase. Its room-temperature effective magnetic moment is 2.89 B.M., indicating a paramagnetic complex with negligible electronic communication between the two Cu(II) centers. Cyclic voltammetry shows two sets of overlapping peaks at -0.33 V and -0.38 V corresponding to Cu(II)Cu(II)-Cu(II)Cu(I) and Cu(II)Cu(I)-Cu(I)Cu(I) indicating diffusion-controlled, quasireversible reaction accompanied by extensive structural reorganizations.

Deep Insights into Heavy Oil Upgrading Using Supercritical Water by a Comprehensive Analysis of GC, GC-MS, NMR, and SEM-EDX with the Aid of EPR as a Complementary Technical Analysis.

Upgrading of heavy oil in supercritical water (SCW) was analyzed by a comprehensive analysis of GC, GC–MS, NMR, and SEM–EDX with the aid of electron paramagnetic resonance (EPR) as a complementary technical analysis. The significant changes in the physical properties and chemical compositions reveal the effectiveness of heavy oil upgrading by SCW. Especially, changes of intensities of conventional EPR signals from free radicals (FRs) and paramagnetic vanadyl complexes (VO2+) with SCW treatment were noticed, and they were explained, respectively, to understand sulfur removal mechanism (by FR intensity and environment destruction) and metal removal mechanism (by VO2+ complexes’ transformation). For the first time, it was shown that electronic relaxation times extracted from the pulsed EPR measurements can serve as sensitive parameters of SCW treatment. The results confirm that EPR can be used as a complementary tool for analyzing heavy oil upgrading in SCW, even for the online monitoring of oilfield upgrading.

Synthesis and characterisation of a range of Fe, Co, Ru and Rh triphos complexes and investigations into the catalytic hydrogenation of levulinic acid

The coordination chemistry of the N-triphos ligand (NP3Ph, 1b) has been investigated with a range of Fe, Co and Rh precursors and found to form either tridentate or bidentate complexes. Reaction of NP3Ph with [Rh(COD)(CH3CN)2]BF4 resulted in the formation of the tridentate complex [Rh(COD)(κ3-NP3Ph)]BF4 (3) in the solid state, however, in solution a bidentate complex predominates in more polar solvents. Reaction of NP3Ph with Fe carbonyl precursors revealed the formation of the bidentate complexes [Fe(CO)3(κ1,κ2-NP3Ph)Fe(CO)4] (4) and [Fe(CO)3(κ2-NP3Ph)] (5), while reaction with FeBr2 resulted in the paramagnetic bidentate complex [Fe(Br)2(κ2-NP3Ph)] (6). Reaction of NP3Ph with CoCl2 gave a dimeric Co species [(κ2-NP3Ph)CoCl(κ1,κ2-NP3Ph)CoCl3] (7), while Zn powder reduction of NP3Ph Co halides resulted in the formation of the tridentate complexes of the type: [Co(X)(κ3-NP3Ph)]. The related triphos Ru complex, [Ru(CO3)(CO)(κ3-CP3Ph)] (2), has also been isolated and characterised. Preliminary catalytic hydrogenation of levulinic acid (LA) was conducted with 2 and 3. The Ru complex was found to be catalytically active, giving high conversions of LA to form gamma-vvvalerolactone (GVL) and 1,4-pentanediol (1,4-PDO), while 3 was found to be catalytically inactive. In situ catalytic testing with 1b and Fe(BF4)2.6H2O resulted in low conversions of LA while a combination of 1b and Co(BF4)2.6H2O gave high conversions to GVL.

Detection of U-87 Tumor Cells by RGD-Functionalized/Gd-Containing Giant Unilamellar Vesicles in Magnetization Transfer Contrast Magnetic Resonance Images.

The targeting of tumor cells and their visualization with magnetic resonance imaging (MRI) is an important task in biomedicine. The low sensitivity of this technique is a significant drawback and one that may hamper the detection of the imaging reporters used.To overcome this sensitivity issue, this work explores the synergy between 2 strategies: (1) arginine, glycine, aspartic acid peptide (RGD)-functionalized giant unilamellar vesicles (GUVs) loaded with Gd complexes to accumulate large amounts of MRI contrast agent at the targeting site; and (2) the use of magnetization transfer contrast (MTC), which is a sensitive MRI technique for the detection of Gd complexes in the tumor region. Giant unilamellar vesicles were prepared using the gentle swelling method, and the cyclic RGD targeting moiety was introduced onto the external membrane. Paramagnetic Gd-containing complexes and the fluorescent probe rhodamine were both part of the vesicle membranes and Gd-complexes were also the payload within the inner aqueous cavity. Giant unilamellar vesicles that were loaded with the imaging reporters, but devoid of the RGD targeting moiety, were used as controls. U-87 MG human glioblastoma cells, which are known to overexpress the targets for RGD moieties, were used. In the in vivo experiments, U-87 MG cells were subcutaneously injected into nu/nu mice, and the generated tumors were imaged using MRI, 15 days after cell administration. Magnetic resonance imaging was carried out at 7 T, and T2W, T1W, and MTC/Z-spectra were acquired. Confocal microscopy images and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) were used for result validation. In vitro results show that RGD GUVs specifically bind to U-87 MG cells. Microscopy demonstrates that (1) RGD GUVs were anchored onto the external surface of the tumor cells without any internalization; (2) a low number of GUVs per cell were clustered at specific regions; and (3) there is no evidence for macrophage uptake or cell toxicity. The MRI of cell pellets after incubation with RGD GUVs and untargeted ctrl-GUVs was performed. No difference in T1 signal was detected, whereas a 15% difference in MT contrast is present between the RGD GUV-treated cells and the ctrl-GUV-treated cells.Magnetic resonance imaging scans of tumor-bearing mice were acquired before and after (t = 0, 4 hours and 24 hours) the administration of RGD GUVs and ctrl-GUVs. A roughly 16% MTC difference between the 2 groups was observed after 4 hours. Immunofluorescence analyses and ICP-MS analyses (for Gd-detection) of the explanted tumors confirmed the specific accumulation of RGD GUVs in the tumor region. RGD GUVs seem to be interesting carriers that can facilitate the specific accumulation of MRI contrast agents at the tumor region. However, the concentration achieved is still below the threshold needed for T1w-MRI visualization. Conversely, MTC proved to be sufficiently sensitive for the visualization of detectable contrast between pretargeting and posttargeting images.

Unusual Formation of the Paramagnetic Complex (η4-C4Me4)CoI2(PhTeI) and Specific Features of Its Electronic, Molecular, and Crystal Structures

The 18-electron (η4-C4Me4)Co(CO)2TeI2Ph compound noticeably decomposes upon prolonged storage and also unusually transforms into the 17-electron (η4-C4Me4)CoI2(PhTeI) complex (I). The X-ray diffraction study of the molecular and crystal structures of complex I (CIF file CCDC no. 1991837) reveals interesting analogies with the molecular structures of the known metal complexes containing organotellurium halide ligands and with the crystal packing of the related one-component crystals of (η4-C4Me4)Co(СО)2TeI2Ph and (η4-C4Me4)Co(СО)2I and two-component crystals of (η4-C4Me4)Co(СО)2I–1,4-C6H4I2. An analysis of the crystal packing of molecules of complex I elucidates a new supramolecular synthone I···(η4-C4Me4) determining the structural motifs of self-assembly in complex I and related compounds (η4-C4Me4)Co(CO)2I, (η4-C4Me4)Co(CO)2I···1,4-C6H4I2, and (η4-C4Me4)Co(CO)2TeI2Ph. The EPR study of compound I shows that cobalt exists in the low-spin state with the total spin S = 1/2.

Solution and solid-state light-induced transformations in heterometallic vanadium-ruthenium nitrosyl complex

The work is devoted to the preparation and photochemical investigations of the novel binuclear heterometallic complex [Ru(NO)(NO2)2(μ-NO2)(μ−CH3COO)(μ-O)VO(dbbpy)]·CH3CN (dbbpy - 4,4′-di-tert-butyl-2,2′-bipyridyl), which is obtained with quantitative yield. The structure of the complex is determined by single crystal X-ray diffraction and by DFT calculations. On the one hand, irradiation at 445 nm of an acetonitrile solution of the complex leads to a NO photo-release reaction. The light excitation induces the charge transfer from NO2 and dbbpy ligands to the antibonding orbitals of ON-Ru-O-V chain, leading to the NO release, which is confirmed by the DFT. The quantum yield of the Ru-NO photo-cleavage is 0.57 ± 0.05 %. The products of the photolysis (NO and paramagnetic RuIII complex) are confirmed by the Griess test and EPR-spectroscopy. On the other hand, the irradiation of the complex in the solid phase at 10 K results in the formation of the metastable bond isomer Ru-ON (MS1), which is detected by the infrared spectroscopy. The stretching vibration of the ν(NO) band of MS1 (1750 cm−1) is shifted by 150 cm−1 to lower energy with respect to the ν(NO) band of the ground state GS (1900 cm−1). The population of the MS1 is about 5%. MS1 thermally decays at 120−140 K back to the GS. Hence, this complex represents a bifunctional platform, which can release nitric oxide in solution and reversibly switch the NO ligand coordination in the solid state. It is shown, that the presence of vanadium strongly influences the photochemical properties of ruthenium nitrosyl both in the solid phase and in solution.

Dy-DTPA as supersensitive shifting and relaxational probe for NMR/MRI control of local temperature

The article develops an approach in which paramagnetic lanthanide complexes are considered and studied as NMR / MRI relaxational and shifting probes for determining local temperature and prospective diagnostics. A detailed NMR study of the paramagnetic properties of Na2[Dy(DTPA)(H2O)] (I) complex is an example of the application of this approach. The substantial temperature dependence of the transverse relaxation on the ligand nuclei at high magnetic fields is due to the Curie spin contribution to the paramagnetic relaxation rate enhancement. A significant temperature dependence of relaxivity r2 was experimentally detected. The activation free energy of the racemization ΔG≠(298) in I is found to be 50 kJ mol−1. The coordination compound I can also serve as the most sensitive 1H NMR relaxational and shifting lanthanide paramagnetic probe for in situ temperature control in liquid medium.

Longitudinal and transverse electron paramagnetic resonance in a scanning tunneling microscope.

Electron paramagnetic resonance (EPR) spectroscopy is widely used to characterize paramagnetic complexes. Recently, EPR combined with scanning tunneling microscopy (STM) achieved single-spin sensitivity with sub-angstrom spatial resolution. The excitation mechanism of EPR in STM, however, is broadly debated, raising concerns about widespread application of this technique. We present an extensive experimental study and modeling of EPR-STM of Fe and hydrogenated Ti atoms on a MgO surface. Our results support a piezoelectric coupling mechanism, in which the EPR species oscillate adiabatically in the inhomogeneous magnetic field of the STM tip. An analysis based on Bloch equations combined with atomic-multiplet calculations identifies different EPR driving forces. Specifically, transverse magnetic field gradients drive the spin-1/2 hydrogenated Ti, whereas longitudinal magnetic field gradients drive the spin-2 Fe. Also, our results highlight the potential of piezoelectric coupling to induce electric dipole moments, thereby broadening the scope of EPR-STM to nonpolar species and nonlinear excitation schemes.

Imaging Tissue Physiology In Vivo by Use of Metal Ion-Responsive MRI Contrast Agents.

Paramagnetic metal ion complexes, mostly based on gadolinium (Gd3+), have been used for over 30 years as magnetic resonance imaging (MRI) contrast agents. Gd3+-based contrast agents have a strong influence on T1 relaxation times and are consequently the most commonly used agents in both the clinical and research environments. Zinc is an essential element involved with over 3000 different cellular proteins, and disturbances in tissue levels of zinc have been linked to a wide range of pathologies, including Alzheimer’s disease, prostate cancer, and diabetes mellitus. MR contrast agents that respond to the presence of Zn2+ in vivo offer the possibility of imaging changes in Zn2+ levels in real-time with the superior spatial resolution offered by MRI. Such responsive agents, often referred to as smart agents, are typically composed of a paramagnetic metal ion with a ligand encapsulating it and one or more chelating units that selectively bind with the analyte of interest. Translation of these agents into clinical radiology is the next goal. In this review, we discuss Gd3+-based MR contrast agents that respond to a change in local Zn2+ concentration.

Systematic Trends in the Electrochemical Properties of Transition Metal Hydride Complexes Discovered by Using the Ligand Acidity Constant Equation.

Understanding the thermodynamics of paramagnetic transition metal hydride complexes, especially of the abundant 3d metals, is important in the design of electrocatalysts and organometallic catalysts. The pKaMeCN([MHLn]+/[MLn) of paramagnetic hydrides in MeCN are estimated for the first time using the ligand acidity constant (LAC) equation where contributions to the pKaMeCN from each ligand are simply added together, with the sum corrected for effects of charge and 5d metals. The pKaLAC-MeCN([MHLn]+/MLn) of over 200 hydride complexes MHLn are used, along with their electrochemical potentials from the literature, in an uncommonly applied thermochemical cycle in order to reveal systematic trends in the redox couples MIII/II and MV/IV (M = Cr, Mo, W), MnII/I, ReVI/V and ReIV/III, MIII/II and MIV/III (M = Fe, Ru, Os), and MIII/II and MII/I (M = Co, Rh, and Ir) and allow the estimation of the bond dissociation free energies BDFE(MH) of the unoxidized hydrides MHLn and the prediction of the electrochemical potential for their oxidation. Density functional theory (DFT) calculations are used to validate the pKaLAC-MeCN values of hydrides of WIII, MnII, FeIII, RuIII, CoII, and NiIII. When a pKaLAC-MeCN is less than zero for a given complex [MHLn]+, the oxidation of MHLn is irreversible due to proton loss from the oxidized complex to the solvent. When pKaLAC-MeCN ≫ 0, the oxidation is reversible when there is no gross change in the coordination geometry upon a change in the redox state. Twenty paramagnetic hydrides prepared in bulk all have pKaLAC-MeCN > 8.

A Paramagnetic NMR Spectroscopy Toolbox for the Characterisation of Paramagnetic/Spin-Crossover Coordination Complexes and Metal-Organic Cages.

The large paramagnetic shifts and short relaxation times resulting from the presence of a paramagnetic centre complicate NMR data acquisition and interpretation in solution. As a result, NMR analysis of paramagnetic complexes is limited in comparison to diamagnetic compounds and often relies on theoretical models. We report a toolbox of 1D (1H, proton‐coupled 13C, selective 1H‐decoupling 13C, steady‐state NOE) and 2D (COSY, NOESY, HMQC) paramagnetic NMR methods that enables unprecedented structural characterisation and in some cases, provides more structural information than would be observable for a diamagnetic analogue. We demonstrate the toolbox's broad versatility for fields from coordination chemistry and spin‐crossover complexes to supramolecular chemistry through the characterisation of CoII and high‐spin FeII mononuclear complexes as well as a Co4L6 cage.

Pressure-Induced Conversion of a Paramagnetic FeCo Complex into a Molecular Magnetic Switch with Tuneable Hysteresis.

A key challenge in the design of magnetic molecular switches is to obtain bistability at room temperature. Here, we show that application of moderate pressure makes it possible to convert a paramagnetic FeIII 2 CoII 2 square complex into a molecular switch exhibiting a full dia- to paramagnetic transition: FeII CoIII ⇔ FeIII CoII . Moreover, the complex follows a rare behavior: the higher the pressure, the broader the magnetic hysteresis. Thus, the application of an adequate pressure allows inducing a magnetic bistability at room temperature with predictable hysteresis width. The structural studies at different pressures suggest that the pressure-enhanced bistability is due to the strengthening of intermolecular interactions upon pressure increase. An original microscopic Ising-like model including pressure effects is developed to simulate this unprecedented behavior. Overall, this study shows that FeCo complexes could be very sensitive piezo switches with potential use as sensors.

Pressure‐Induced Conversion of a Paramagnetic FeCo Complex into a Molecular Magnetic Switch with Tuneable Hysteresis

AbstractA key challenge in the design of magnetic molecular switches is to obtain bistability at room temperature. Here, we show that application of moderate pressure makes it possible to convert a paramagnetic FeIII2CoII2 square complex into a molecular switch exhibiting a full dia‐ to paramagnetic transition: FeIICoIII ⇔ FeIIICoII. Moreover, the complex follows a rare behavior: the higher the pressure, the broader the magnetic hysteresis. Thus, the application of an adequate pressure allows inducing a magnetic bistability at room temperature with predictable hysteresis width. The structural studies at different pressures suggest that the pressure‐enhanced bistability is due to the strengthening of intermolecular interactions upon pressure increase. An original microscopic Ising‐like model including pressure effects is developed to simulate this unprecedented behavior. Overall, this study shows that FeCo complexes could be very sensitive piezo switches with potential use as sensors.

Paramagnetic Cobalt(II) Complexes with Cyclam Derivatives: Toward 19F MRI Contrast Agents.

In order to develop novel, more efficient, and/or selective contrast agents for magnetic resonance imaging (MRI), different modi operandi are explored as alternatives to water-relaxation enhancement. In this work, cobalt(II/III) complexes of bis(N-trifluoroethyl)cyclam derivatives with two acetate or two phosphonate pendant arms, H2te2f2a and H4te2f2p, were prepared and investigated. X-ray diffraction structures confirmed octahedral coordination with a very stable trans-III cyclam conformation and with fluorine atoms located about 5.3 Å from the metal center. The Co(II) complexes are kinetically inert, decomposing slowly even in 1 M aqueous HCl at 80 °C. The Co(II) complexes exhibited well-resolved paramagnetically shifted NMR spectra. These were interpreted with the help of quantum chemistry calculations. The 13C NMR shifts of the trans-[CoII(te2f2p)]2- complex were successfully assigned based on spin density delocalization within the ligand molecule. The obtained spin density also helps to describe d-metal-induced NMR relaxation properties of 19F nuclei, including the contribution of a Fermi contact relaxation mechanism. The paramagnetic complexes show convenient relaxation properties to be used as 19F MRI contrast agents.

Complexes of the Bis(di‐tert‐butyl‐aniline)amine Pincer Ligand: The Case of Copper

The N,N‐bis(2‐amino‐3,5‐di‐tert‐butylphenyl)amine pincer ligand was coordinated to copper. Depending on the copper source, a mononuclear complex 1+ or a trimer 2 could be isolated and were structurally characterized. Complex 1+ consists of two deprotonated iminobenzoquinone ligands coordinated to a Cu(I) center. Complex 2 is trinuclear with a (3:3) (M:L) stoichiometry, featuring a three‐fold repetition of a unit made of a Cu(II) center coordinated to a tridentate ligand radical‐dianion. In 2, the metal ions are bridged by an anilido nitrogen. The coordination sphere of each copper is completed to four by a neighboring iminosemiquinone moiety. Complex 1+ belongs to an electron‐transfer series. The paramagnetic complexes 1 and 12+ were generated and characterized by EPR and Vis‐NIR spectroscopy. Complex 1 exhibits an isotropic resonance at g = 2.00, which is reminiscent of Cu(I) iminosemiquinone species. The dication 12+ exhibits a metal‐based ground spin state and hence is described as a Cu(II) iminobenzoquinone complex. Both 1 and 1+ show a NIR band (954, 980 nm) of high intensity (> 20 mm–1 cm–1) assigned to ligand‐based charge transfer transitions. Two‐electron reduction of 1 produces 2 via ligand release and disproportionation. Conversely, oxidation of 2 affords 1+. Finally, carbon‐nanotube‐supported complex 2 is active towards electrocatalytic reduction of H2O2.

Paramagnetic Pyrazolylborate Complexes Tp2M and Tp*2M: 1H, 13C, 11B, and 14N NMR Spectra and First-Principles Studies of Chemical Shifts.

The paramagnetic pyrazolylborates Tp2M and Tp*2M (M = Cu, Ni, Co, Fe, Mn, Cr, V) as well as [Tp2M]+ and [Tp*2M]+ (M = Fe, Cr, V) have been synthesized and their NMR spectra recorded. The 1H signal shift ranges vary from ∼30 ppm (Cu(II) and V(III)) to ∼220 ppm (Co(II)), and the 13C signal shift ranges from ∼180 ppm (Fe(III)) to ∼1150 ppm (Cr(II)). The 11B and 14N shifts are ∼360 and ∼730 ppm, respectively. Both negative and positive shifts have been observed for all nuclei. The narrow NMR signals of the Co(II), Fe(II), Fe(III), and V(III) derivatives provide resolved 13C,1H couplings. All chemical shifts have been calculated from first-principles on a modern version of Kurland-McGarvey theory which includes optimized structures, zero-field splitting, and g tensors, as well as signal shift contributions. Temperature dependence in the Fe(II) spin-crossover complex results from the equilibrium of the ground singlet and the excited quintet. We illustrate both the assignment and analysis capabilities, as well as the shortcomings of the current computational methodology.

Analysis of the Relaxometric Properties of Extremely Rapidly Exchanging Gd3+ Chelates: Lessons from a Comparison of Four Isomeric Chelates.

Relaxometric analyses and in particular the use of fast-field cycling techniques have become routine in the study of paramagnetic metal complexes. The field dependence of the solvent proton relaxation properties (nuclear magnetic relaxation dispersion, NMRD) can provide unparalleled insights into the chemistry of these complexes. However, analyzing NMRD data is a multiparametric problem, and some sets of variables are mutually compensatory. Specifically, when fitting NMRD profiles, the metal-proton distance and the rotational correlation time constant have a push-pull relationship in which a change to one causes a predictable compensation in the other. A relaxometric analysis of four isomeric chelates highlights the pitfalls that await when fitting the NMRD profiles of chelates for which dissociative water exchange is extremely rapid. In the absence of independently verified values for one of these parameters, NMRD profiles can be fitted to multiple parameter sets. This means that NMRD fitting can inadvertently be used to buttress a preconceived notion of how the complex should behave when a different parameter set may more accurately describe the actual behavior. These findings explain why the effect of very rapid dissociative exchange on the hydration state of Gd3+ has remained obscured until only recently.

Different flavors of diffusion in paramagnetic systems: Unexpected NMR signal intensity and relaxation enhancements

The NMR community is well acquainted with different kinds of diffusion but, at the same time, there are several effects that are worth a better understanding for an improved design of molecular imaging and dynamic nuclear polarization experiments. Spin diffusion and chemical diffusion are known to play important roles in determining the NMR signal and relaxation enhancements caused by the presence of paramagnetic molecules in solution. Paramagnetic complexes are used as contrast agents in magnetic resonance imaging, due to their efficacy in selectively increase the relaxation rates of solvent water protons, as well as in dynamic nuclear polarization experiments to increase the NMR signal of desired molecules through polarization transfer from unpaired electrons. In this paper we review some recent, unexpected observations in these two areas, which seem related to spin and/or chemical diffusion, and demonstrate the need for a detailed understanding of the interplay of different phenomena. A deeper understanding of spin and chemical diffusion may thus result very important for an improved design of contrast agents for magnetic resonance imaging and for the optimization of hyperpolarization experiments.